Beneficiation process to produce low ash clean coal from high ash coals

ABSTRACT

A beneficiation process to produce low ash clean coal from high ash coals, including: preparing a coal slurry from fine coal and water and discharging it to a first reactor; preparing an alkali solution and discharging it to a first reactor; carrying-out a leaching reaction inside the first reactor; transferring the reaction mixture to a second reactor for filtration and washing to produce a filter cake; preparing a diluted acid solution and delivering it to a second reactor; preparing a diluted alkali solution and delivering it to a second reactor; feeding a coal slurry prepared from the filter cake into the second reactor for washing and transferring to a second filtration unit; transferring the product after filtration to a third reactor; carrying-out different leaching reaction sequences in the first, second, and third reactors; and transferring to a fifth tank the treated coal filter cake for drying.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the United States national phase of InternationalApplication No, PCT/IN2009/000328 filed Jun. 5, 2009, and claimspriority to Indian Patent Application No. 1518/KOL/08, flied On Sep. 3,2008, the disclosures of which are hereby incorporated by reference intheir entireties.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an improved beneficiation process toproduce low ash clean coal from high ash coals. The invention furtherrelates to a system for implementing an improved beneficiation processto produce low ash clean/coal from high ash coals.

2. Description of Related Art

Coal and coal products continue to play an increasingly important rolein fulfilling the energy needs of our society. However, it is generallyknown that most of the raw coals are associated with mineral matter,which makes it unsuitable for efficient utilization, such ascarbonization, gasification, combustion, or liquefaction. Hence,demineralization of the raw coal has tremendous application in themetallurgical industries, thermal power plants and other industries,which require coals with low ash content. Accordingly, physical as wellas chemical coal cleaning (beneficiation) processes have been explored.In general, physical coal cleaning processes involve pulverizing thecoal to remove the impurities, wherein the fineness of the coalgenerally governs the degree to which the impurities are released.Although, the cost of preparing the coal exponentially increases withthe amount of fine to be treated, there however exists an economicoptimum in size reduction. Although, a step of grinding coal even toextremely fine sizes may not be effective in removing all theimpurities. Based on the physical properties that effect the separationof the coal from the impurities, physical coal cleaning methods aregenerally divided into four categories for example, gravity, flotation,magnetic and electrical.

As compared to the techniques of physical coal cleaning, chemical coalcleaning techniques are in a very early stage of development. As far asIndian scenario is concerned, most of the coals present in India are ofdrift origin and low grade having high mineral matter contents varyingfrom 5% to greater than 50%. Moreover, the mineral matter is finelydisseminated inside the coal matrix and is at times firmly bound. Again,since washability characteristics of Indian coal is not good, it isdifficult to remove the mineral matter from coal by conventionalphysical coal washing techniques based on specific gravity differencesuch as heavy media and dense media etc. Hence, physical methods ofbeneficiation such as heavy media and froth flotation, are of limiteduse for applications in coal beneficiation to produce low ash coals formetallurgical coke making and power generation.

Chemical leaching of coal is a fairly-known technology to produce ultraclean where the ash content of the clean is as low as ˜1.0% based onmineralogical composition of the feed coal. There are potential use ofthis ultra clean coal both as a fuel and nonfuel.

A prior art process reveals that German coal processing plants treatedcoal with aqueous sodium hydroxide at elevated temperatures andpressures, and thereafter the coal was extracted with aqueoushydrochloric acid. The said process reduced the sulphur and ash contentof the coal. (Crawford, BIOS Final Report No. 522, Item No. 30, Feb. 19,1946, British Intelligence Objectives Sub-committee, London(A.T.I.—118668, Central Documents Office Wright-Patterson Airforce Base,Dayton, Ohio). Subsequently, the U.S. Bureau of Mines evaluated asimilar process for treating coal, involving leaching with aqueoussodium hydroxide at 225° C., both with and without a final stageextraction with aqueous hydrochloric acid. In a report made by Reggel etal in 1972, it was concluded that the sequence of sodium hydroxidetreatment and hydrochloric acid extraction removed most of the mineralmatter originally present in the coal. Am. Chem. Soc. Div. of Fuel Chem.Preprints, 17(1): 44-48. Battelle Memorial Institute had developed asimilar process, which is described in Stambaugh et al U.S. Pat No.4,055,400 of 1977. According, to the said disclosure, an aqueousalkaline slurry of coal is heated at an elevated temperature andpressure to leach out sulphur and mineral matter. The Battelle processmay optionally include last stage extraction with aqueous acid to reducethe final ash content. (Stambaugh et al, Hydrocarbon Processing, 54 (7):115-116 (1975)). Another known process has undergone extensivedevelopment at Iowa State University, Ames, Iowa.

The “Ames” process uses oxidative desulphurization in aqueous slurry ofsodium carbonate. Typical conditions are 0.2M Na₂CO₃ at an oxygenpartial pressure of about 4 atmosphere and temperature of 120-140 deg.C. for 1-2 hrs. This development was reviewed in detail by Dr. T. D.Wheelock in 1981. (Chem. Eng. Commun., 12;137-159). In onerepresentative test, using temperature of 120-140 deg. C., the totalsulfur content of the coal was reduced by 70% and the pyretic sulphurcontent was reduced by 78%. (Wheelock (1981).

The Japanese patent publication 466/1942 describes a process forremoving ash from coal or coke. The Japanese patent publication23711/1971 discloses a process for removing sulfur and ash from coals.The Japanese patent publication 133487/1980 describes a coal deashingprocess. The processes of the first and second of the above-saidJapanese publications, use an acid or alkali with the application ofpressure and heat to dissolve the metallic components for the removal ofash. When practiced under moderate conditions, these processes failalmost to achieve any ash removing effect and are therefore not suitableas the deashing processes. The process disclosed in the third Japanesepublication, the oxidation is followed by an acid or alkali treatmentakin to the first and second processes and is such that the FeS₂components which are difficult to dissolve are first oxidized andthereafter dissolved. A further prior art provides use of hydrofluoricacid or hydrogen fluoride gas for treatment of coal since SiO₂ is noteasily soluble in acids or alkalis to separate Si in the form of gaseousSiF₄ to achieve a deashing effect. However, the use of hydrofluoric acidor hydrogen fluoride gas, which is highly toxic and corrosive, involvesmany difficulties. Thus an actually effective and useful process forremoving ash from coal still remains to be developed although thedeashing of coal is admittedly a very important technique for theeffective use of coal.

The prior art also describes several chemical coal beneficiationprocesses, for example, U.S. Pat. No. 4,424,062 discloses a process forchemically removing ash coal by immersing ash containing coal in anaqueous solution containing hydrochloric acid or citric acid incombination with acidic ammonium fluoride. U.S. Pat. No. 3,993,455discloses a process for removing mineral matter from coal by treatmentof the coal with aqueous alkali such as sodium hydroxide, followed byacidification with strong acid. Similarly, U.S. Pat. No. 4,55,400discloses a method of extracting sulphur and ash from coal by mixing thecoal with an aqueous alkaline solution, such as ammonium carbonate.

U.S. Pat. No. 4,071,328 discloses a method of removing sulphur from coalby first hydrogenating the coal and the hydrogenated coal wassubsequently contacted with an aqueous inorganic acid solution. U.S.Pat. No. 4,127,390 discloses a process for reducing the sulphur contentof coal by treatment with an aqueous sodium chloride solution. U.S. Pat.No. 4,134,737 discloses a process for the production of beneficiatedcoal wherein the coal is digested in caustic, then treated in mineralacid and then treated in nitric acid.

U.S. Pat. No. 4,083,940 discloses a process for cleaning coal bycontacting the coal with an aqueous leaching solution containing nitricand hydrofluoric acid. U.S. Pat. No. 4,169,710 discloses comminuting andcleaning coal of sulphur and ash by contacting the coal with a hydrogenhalide, such as HF (aqueous and/or anhydrous).

U.S. Pat. No. 4,408,999 discloses beneficiating coal by subjecting thecoal to electromagnetic radiation in the presence of a strong inorganicacid, such as hydrofluoric acid. U.S. Pat. No. 4,305,726 discloses achemical method of removing ash and sulphur from coal, the methodcomprising treating the coal with hydrochloric and hypochlorous acid inthe presence of ferric and ferrous sulphate. U.S. Pat. No. 4,328,002dislcoses a method of treating coal to remove ash and sulphur involvingpreconditioning coal particles in the presence of an aqueous solution ofan oxidant, such as H₂O₂ or HF, washing the so-treated coal, treatingthe washed coal with further oxidant and then passivating the coal withfor example, an ammonium salt and then neutralizing with alkali metalhydroxide.

U.S. Pat. No. 4,516,980 discloses a process for producing low-ash, lowsulphur coal by a two-stage alkaline treatment using sodium carbonate orbicarbonate as the reagent. The alkaline treated coal is then extractedwith aqueous mineral acid; and U.S. Pat. No. 3,998,604 discloses a coaldemineralization process whereby ground coal is treated with aqueousacid, such as HCl, H₂SO₄ or H₂CO₃ and then subjected to froth flotationin the presence of a gas selected from Cl₂so₂ or CO₂.

All prior art described hereinabove, mostly entail high cost interaliahigh consumption of energy. Further, the prior art normally deals withcoal containing moderately high ash-content coal but not like Indiancoals which contain upto 50% of high mineral matter.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to propose animproved beneficiation process to produce low ash clean coal from rawcoal, which eliminates the disadvantages of prior art.

Another object of the present invention is to propose an improvedbeneficiation process to produce low ash clean coal from raw coal, whichis economic and commercially viable.

A further object of the present invention is to propose an improvedbeneficiation process to produce low ash clean coal from raw coal, whichconsume less power.

A still further object of the present invention is to propose animproved beneficiation process to produce low ash clean coal from rawcoal, which is eco-friendly.

A still another object of the present invention is to propose a systemfor implementing an improved beneficiation process to produce low ashclean coal from raw coal.

According to the invention the ash content of high ash Indian coals canbe reduced up to ˜4.0-5.0% in the produced clean coal. In the presentinvention, an improved process and system is provided for treating highash Indian coals through a series of alkali and acid treatment stepsunder different operating conditions to produce low ash coal. Theinventive method is implemented by a system envisaged to operate on abatch production process. Each batch is capable to process 500 kg of rawcoal depending on the system configuration. In the case of chemicalleaching processes for removing ash from coal, the inorganic substancesconstituting the ash content of coal are reacted with chemical agentsand separated from the coal for removal. Various chemicals are used forthe chemical beneficiation process. Some of these chemicals normallyhave a tendency to differentially dissolve certain inorganicconstituents compared to the others and the actual chemical to be usedmay depend upon the inorganic content of the carbonaceous material whichis fed to the process.

Leaching or solid extraction is done to dissolve mineral matter in coalusing a solvent. The acidic and/or basic components present in mineralmatter react with the solvents, get dissolved and then removed.According to the invention, the coal is either crushed to −30/−72 BSmesh size to produce fine clean coal or fine clean coal is obtained fromflotation circuit coal, and thereafter the ash-forming minerals areremoved. The removal step comprises a series of alkali and acidtreatment steps under various operating conditions. The steps includetreatment of the feed coal in an aqueous alkaline solution at anelevated temperature under atmospheric and elevated pressures followedby reaction/extraction with an aqueous acidic solution at atmospherictemperature and pressures. The inventive process produces low ash(˜4.0-5.0% ash) clean coal from high ash Indian coals with 75-85% yield.The present invention is a step towards making chemical leaching processcommercially feasible for various applications.

Accordingly, there is provided an improved beneficiation process toproduce low ash clean coal from high ash coals, comprising the steps offeeding raw coal to a continuous ball mill with size reduction via aprimary crusher, screening the crushed coal outputted from the ballmill; transferring the crushed and screened coal (fine coal) to a bunkerhaving load cells for weight-monitoring of the fine coal including avibratory device for smooth inflow, and a rotary feeder device for easyoutflow from the bunker; preparing a coal slurry from the fines receivedin batches from the bunker in a first tank having means for stirring,and provided with process water controlled via a control valve, the tankbeing further supplied with compressed air, the prepared coal slurrybeing discharged from the tank to a first reactor, the first reactorbeing maintained at a temperature between 85 to 90° C.; preparing analkali solution with a predetermined concentration with NaOH and waterin a second tank; and discharging the prepared alkali solution to saidfirst reactor; carrying-out a leaching reaction between the coal and theaqueous alkali solution inside the first reactor for about a periodbetween 2 to 5 hours by varying the speed of the stirrer inside thefirst reactor; transferring the reaction mixture from the first reactorto a second reactor either directly or through a first rotary drumfilter for filtration and washing to produce a filter cake including afiltrate, the filtrate being transferred to a separate storage tank, thefilter cake being continuously washed by using sprayed water andtransferred back to the coal slurry preparation tank; preparing adiluted acid solution in a second tank by supplying therein concentratedacid and process water via feed lines having control valves and flowsensors, the second tank being provided with at least one level sensor,and at least one stirrer, the prepared acid solution being delivered toa second reactor, the second reactor having a stirrer; preparing adiluted alkali solution in a third tank having atleast one first feedline for delivery of alkali, recycled alkali with load cells, controlvalves, and flow sensors, the tank having level sensors includingstirrer, and atleast one second feed line for supply of process water,the produced alkali solution including recycled alkali being deliveredto the second reactor; preparing a slaked lime in a fourth tank foradding the slaked lime with the recycled alkali alongwith with the freshalkali; feeding the coal slurry prepared from said filter cake into thesecond reactor for washing the alkali-treated coal with acid at roomtemperature, and transferring to a second filtration unit forfiltration; transferring the product after filtration to a thirdreactor; carrying-out different leaching reaction sequence in the first,second, and third reactors; transferring the treated slurry to a secondfiltration unit after completion of the reaction from the third reactor;transferring to a fifth tank the finally treated coal filter cake fordrying, the dried filter cake being taken up for physical, chemical,rheological, petroggraphical analysis; and neutralizing the filtratedischarged from the second filtration unit with lime, the rest of thefiltrate being transferred to an evaporator to obtain the requisiteconcentration of NaoH.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow sheet of the inventive Chemical Leaching process ina system according to the invention.

FIG. 2 shows a block flow diagram of the system for implementingChemical Leaching process of Coal according to the invention.

FIG. 3 shows a layout of the system adaptable for implementing aChemical Leaching process of Coal according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Process Units

The system for implementing the inventive process is shown in FIG. 1.The system is divided into 5 zones, each zone serving the specificpurpose of facilitating the various requirements for the up-gradation ofthe existing system. The block flow diagram which shows the five zonesis depicted in FIG. 2. The proposed layout for the system is shown inFIG. 3.

As shown in FIG. 2, Zone 1 consists of a Raw material storage area (1)in the form of a covered shed which includes an area for storing rawcoal, an area for storage of finished product, and a room for chemicalstorage (2).

The raw coal received from the mine is stored in a first part of thecovered shed (1). A storage facility is made available for storing coal.The final product is stored in a second part of the covered shed (1).

A separate room is made available within the covered shed for storingthe chemicals (2). Amongst the chemicals, the acids are stored in cansor drums, and the alkali is stored in racks as it is supplied in bags.

A limekin (9) is provided to generate fresh lime powder from thelimestone. The limestone in the form of raw material is kept in an openarea. The fresh lime from the kiln (9) is stored in covered containers.Alternatively, fresh lime form external sources can also be arranged andused.

FIG.—2 further shows the zone—2 which consists of two major unitsnamely,

-   -   a coal preparation unit (3); and    -   a feed preparation unit (4)

Further description of the two units (3, 4) are provided hereinbelow:

a) Coal Preparation Unit (3)

The coal preparation unit (3) consists of coal conveyors, a ball mill(19) to pulverize the coal to −72 mesh (0.025 mm), a plurality ofscreens to separate out −72 mesh powders, and several powder storagebunkers.

The raw coal of size 0-25 mm from the storage area is provided into anumber of ground hoppers and lifted to an elevation of atleast 15 m withthe help of a lifting means for example, a plurality of bucket elevatorsand fed to a continuous ball mill (19). If the input size is larger than25 mm, then a primary crusher to reduce the size to 0-25 mm is provided.The primary crusher when provided is disposed at the ground level.

The ball mill (19) crushes the coal to −72 mesh and the output from theball mill (19) is screened to separate the −72 mesh material using thescreens. The under size material is stored into a storage bunker whilethe over size material is re-circulated back to the ball mill (19).

The total capacity of the coal preparation unit (3) is variable but canbe selected in the range 5-10 tph. The fine coal is stored in a bunker.The bunker is provided with load cells to continuously monitor theweight of the powder while filling and also during the discharge. Thebunker is provided with a known vibratory equipment to facilitate thesmooth flow of the fine material from the bunker, during discharges. Thefine powders from the bunker is discharged through a rotary feederdevice.

If the moisture content of the coal is high, namely, the inherentmoisture or the external moisture (i.e. >5%), then a rotary dryer isprovided to dry the coal before it is screened. This drier reduces thepossibility of sticking and choking of the screens and the bunkers dueto high moisture content of the coal.

The coal preparation unit (3) normally can be operated once a day, ifthe requirement to generate 2 T materials per day which caters foratleast for batch operation of 500 kg each.

b) Feed Preparation Unit (4)

The feed preparation unit (4) consists of the following submits:

-   -   a coal slurry preparation submit (5) of atleast 2 m³ capacity,    -   an acid preparation submit (6) of atleast 2 m³ capacity,    -   an alkali preparation submit (7) of atleast 3 m³ capacity, and    -   a slaked lime preparation submit (8) of atleast 2 m³ capacity.

All the submits (5, 6, 7, 8) of the feed preparation unit (4) constitutea single walled circular tank fitted with stirring and agitation meansof corresponding capacity. While the subunit for coal slurry preparation(5) is enabled to withstand abrasion of the coal powders and the otherunits to withstand the acid (6), and alkali (7). The lime units (8) areconfigured as acid, alkali and corrosion resistant. All the associatedpipe lines, feed pumps, delivery pumps; valves (both gate valves and thecontrol valves) are selected to be resistant to acid, alkali andcorrosion.

The feed preparation tanks for the coal slurry (5), alkali (7), and theslaked lime (8) have suitable inlets for feeding the fine powders,including a gate valve for the flow control. The acid preparation tank(6) has suitable inlet for feeding the concentrated acids into the tank.Each of the tanks (5, 6, 7, 8) comprises a dilution water inlet, and acontrol valve for flow control. Each of the tanks is provided withatleast one level sensor for control purposes. Each of the tanks is alsoprovided with a manually operable sample collection point.

1) Coal Slurry Preparation Unit (5):

The coal slurry preparation tank may be selected as a circular tank ofaround 2 m³ capacity. The tank is fitted with a stirring mechanism tostir the material while the slurry is being prepared. The slurryprepared may be in the form of dough or cake or thick slurry and henceheavy capacity stirrers are provided.

The coal slurry preparation tank (5) has an inlet pipe for feeding thecoal powder into the tank. The feed is controlled through a gate valve,which also acts as an isolation valve for the sealing purpose. Theprocess water is fed to the tank and the feed line is provided with acontrol valve. The tank (5) is further provided with level indicatorsfor control purpose.

The coal slurry prepared in the tank (5) may be lime dough or cake orthick slurry and requires forced discharge from the tank (5). Acompressed air line (20) is provided to pressurize the tank (5) tofacilitate discharging of the slurry out of the tank (5). The outlet ofthe tank is provided with a gate valve. The valve also acts as anisolator of the tank apart from controlling the flow.

The output from the coal slurry tank (5) is transported to the reactorzone.

The coal powder from the storage bunker (1) is fed to the slurrypreparation tank (5), in batches of 500 kg coal powder. The feeding maybe controlled by both the load cell data at the coal powder storagebunker (1) and also by the level sensor of the slurry tank (5). Once thecoal powder is taken into the tank (5), the process water (10) is fed tothe tank (5) in a controlled manner to make the slurry of requiredcomposition. The flow of water is controlled through the flow controlvalve provided on the tank (5), and the level sensor signal. The mixtureis continuously stirred while the material is being added.

Once the slurry is made, the outlet gate valve is opened and the outflowof the slurry form the tank (5) is transferred by using the compressedair (20) which ensures complete emptying of the tank (5).

2) Acid Preparation Subunit (6):

The tank (6) used for the acid preparation is provided with an inlet forthe feed, an inlet for the process water (10), a stirrer, and an outlet.Each of the feed lines for the acid, water and the outlet may beprovided with control valves to regulate the feed inlet and also thepipe lines may be provided with flow sensors. The tank (6) is providedwith level sensors. The acid inlet and the outlet of the tank (6) iseach provided with pumps to feed and evacuate the material.

The tank, valves, pumps, flow and level sensors are selected to be acidresistant. The concentrated acid from the storage tanks in the storagearea (2) is pumped to the acid preparation tank (6) in the requiredquantities for preparation of diluted acids with various degrees ofdilutions. The process water (10) is used to prepare the necessarydiluted acids. The contents in the tank (6) are stirred while thediluted acids are prepared.

The final diluted acid is then pumped with the help of pumps to a secondreactor and the flow is controlled through the control valves in theoutlet pipe line.

3) Alkali Preparation Subunit (7):

The tank (7) used for the alkali preparation is provided with a hoppermeant for the feed alkali, a plurality of inlet pipes for the processwater (10), including the recycled alkali (21), and an outlet for thealkali solution. The hopper is further provided with load cells formeasuring the amount of alkali entering into the system so thatrequisite alkali concentration can be maintained inside the tank (7).Each of the feed lines for water (10) including that of the outlet maybe provided with control valves to regulate the feed inlet. The pipelines may be provided with flow sensors. The tank (7) is provided withlevel sensors. The tank (7) outlet is provided with pumps to feed andevaluate the material.

The tank, valves, pumps, flow and level sensors are all alkaliresistant. The concentrated alkali from the storage area (2) is chargedthrough the hopper to the alkali preparation tank (7) in the requiredquantities for preparation of diluted alkali with various dilutions. Theprocess water (10) is used to prepare the diluted alkali. The contentsin the tank (7) are stirred with a stirrer while the diluted alkali isbeing prepared.

The final diluted alkali is then pumped with the help of pumps to asecond reactor and the flow is controlled through the control valves inthe outlet pipe line. In addition to the fresh alkali, recycled alkalisare also added to the second reactor.

4) Slaked Lime Preparation Subunit (8):

The tank (8) used for the slaked lime preparation is provided with ahopper for the feed, an inlet for the process water, a stirrer, and anoutlet. The hopper is provided with load cells for measuring the amountof lime entering into the system so that requisite lime concentrationcan be maintained inside the tank (8). Each of the feed lines for water(10) and the outlet may be provided with control valves to regulate thefeed inlet. The pipe lines may be provided with flow sensors. The tankis provided with level sensors. The tank (8) outlet is provided with apump to feed and evacuate the material.

The tank, valves, pumps, flow and level sensors are selected to becorrosion resistant.

The lime from the storage area (2) is charged through the hopper to theslaked lime preparation tank (8), in the required quantities forpreparation of lime with various dilutions. Process water (10) is usedto prepare the necessary slaked lime. The contents in the tank (8) arestirred while the lime is being prepared.

The final slaked lime is then added to the recycled alkali toprecipitate out calcium silicates. The precipitate is prepared inside athickener and the regenerated alkali is recycled back and charged to athird reactor along with fresh alkalis (7).

Methodology to be Followed for Chemical Leaching of Minerals in a Systemof the Invention

Description for Chemical Leaching Process

Coal Sample Preparation

The feed can be fine clean coals generated in coal washeries throughfroth flotation process or a middling product. The feed sample can betaken directly as obtained after froth flotation treatment if the sizerequirement in −30 BS mesh or can be crushed to −72 BS mesh size (0-025mm) for improving the kinetics of the process. The coal samples areanalysed for their ash content before processing. The major mineralsnormally found in coals are silicates or shales, quartz and/orsandstone, pyrites and carbonates such as siderites and ankerites. Ithas been found that even at 373 deg. K using diluted to moderatelyconcentrated NaOH solution, kaolin is converted into a crystallinesodium derivative i.e. Na₂O.Al₂O₃.I.8SiO₂. The solubility of thissodium-aluminium-silicate derivative is not very high in alkalisolutions but it is fairly soluble in diluted alkali followed by washingwith mineral acid.

Chemicals:

Commercially available sodium hydroxide (NaOH) pellets in combinationwith hydrochloric acid (HCl) are to be used in the present method.Diluted aqueous NaOH solution of 10-50% concentration and 10-20% HCl isprepared and used for the chemical leaching process described below:

The Treatment Process:

Feed coal is collected form the coal storage (1) and fed into a hopperusing a bucket elevator. The coal is allowed to enter into a ball mill(9) from the hopper in which it is crushed to desired size range byallowing sufficient residence time. The crushed coal is then be screenedto obtain −30 BS mesh and/or −72 BS mesh size. Oversize coal at the topof the screen is recycled back to the ball mill (19). The product fromthe screen is stored in a storage hopper around of 2 t capacity.

The coal powder from the storage hopper is fed into the slurrypreparation tank (5). In the slurry preparation tank (5), 500 kg of feedcoal is mixed with 1000-3000 litres of water using an agitator. The coalslurry is then pumped to a first reactor (11). In the other tank (7),alkali solution of a particular concentration is prepared (500-2500 kgNaOH in 2000-4000 litres of water) so as to maintain aqueous alkaliconcentration of 10-50% inside the first Reactor (11). The coal slurryas well as the prepared alkali solution is fed into the first reactor(11) of capacity around 7.0 Nm³. The first reactor (11) is a jackedreactor having an agitator in which temperature up to ˜100° C. can beachieved at atmospheric pressure. In all the above preparation tanks,(5, 6, 7, 8) the agitator speed is maintained at around 200 rpm, tofacilitate proper mixing.

In the first Reactor (11), the temperature is maintained at 85-90° C. byusing saturated steam inside the jacket. The leaching reaction of coalwith aqueous NaOH solution is carried out inside the first reactor (11)for a period of nearly 2 to 5 hr based on requirement of reduction inmineral matter percentage in coal and for achieving optimum leachingsequence in the series of reactors. The stirrer speeds inside thereactors can be varied using a variac for homogenous mixing of the coalslurry with the NaOH solution. A condenser is mounted at the top of thefirst reactor (11) for continuous reflux. After completion of reactionin the first reactor (11), the reaction mixture can be sent to a secondreactor (12) directly or through a first rotary drum filter (13) forfiltration under vacuum and washing.

The filter cake that is formed on the filter cloth of the first rotarydrum filter (13) is continuously washed with wash water (10) so as toremove the silicates and other products those are formed as a result ofreaction of the mineral matter constituents with NaOH. Water washing isdone by spraying water (10) from the top of the first rotary drum filter(13). The filter cake that comes out of the first rotary drum filter(13) is sent for preparation of coal slurry (16). Filtrate coming out ofthe first rotary drum filter (13) is pumped to a separate storage tank.

The coal slurry (16) prepared from the filter cake is then pumped to thesecond reactor (12) or an acid reactor (14). A pressure of 10 kg/cm² ismaintained inside the second reactor (12) by circulating compressed air(20) into the reactors (12, 14). At the highest pressure, a temperatureof nearly 180° C. can be reached by injecting steam (15) through thejacket. Inside the first Reactor (12), 150-170° C. temperature ismaintained. Here, the stirrer speed inside the first reactor ismaintained at around 200 rpm. A condenser/cooling coil arrangement ispresent at the top of the first reactor (12) for continuous reflux.After completion of reaction time, the product from the first reactor(12) is again sent back to the first filtration unit (13) forfiltration. Filtration is carried out in the similar way.

The slurry prepared using the filter cake can also be fed into the acidReactor (14) which is a reactor for treating coal slurry with 5-20% ofHCI at room temperature and atmospheric pressure. This acid reactor (14)is used for washing/treating alkali treated coal with acid so that mostof the dissolved silicates and other reaction products gets washed away.Here also, the stirrer speed inside the acid reactor (14) is maintainedat around 200 rpm. Then the acid treated coal is sent to a secondfiltration unit (18) where filtration is carried out. Product afterfiltration from the second Reactor (12) or acid reactor (14) based onsequence maintained may then be sent to a third Reactor (17) if requiredwhich is an acid reactor similar to the Reactor (14). There is anarrangement for maintaining different leaching sequence i.e.alkali-acid-alkali-acid as well as alkali-alkali-acid-acid using thefour reactors.

After completion of reaction in the third reactor (17), the slurry (16)is sent to the second filtration unit (18) (rotary drum filter 2).Filtration is carried out in the same way, as was done using the firstfiltration unit (13). The coal filter cake obtained after the finaltreatment is stored inside a fifth tank and after drying the same issent for various physical, chemical, rheological, petrographical andother special analysis. The alkali and acid filtrates are sent to twodifferent storage tanks.

The filtrate coming out from the filter (18) after alkali treatment isneutralized with lime to precipitate out the silicates and otherundesirable constituents. The rest of the filtrate which contains mostlypure NaOH is sent to a triple effect evaporator (23) to obtain requisiteconcentration of NaOh. The concentrated NaOH solution (coming out of theevaporator) is pumped back to a recyclable NaOH storage tank from whichit is again charged into the reactor along with fresh alkali solutions.Similarly, the filtrate coming out after the acid treatment is recycledback to a recycle acid storage tank. After particular number of testswhen the acids get contaminated with impurities, the spent acid isdischarged into a storage tank in the product disposal section fromwhich it is sent to a tanker and disposed off in a safe place.

Requirement of Services:

Approximately 500 kW peak power requirement is estimated for the totalplant operation. No emergency power facility is proposed since theoperations are batch processes. Approximately 30,000 litres of 25-cubicm of water (10) requirement is estimated per day for the steamgeneration (15), process as well as wash water (10). A water storagetank of suitable capacity is provided to facilitate uninterrupted supplyof water to the various units. The water is selected to be soft havingnot more than 10 ppm hardness. If the water available is hard, the sameis provided with suitable softening equipment.

Steam (15) is used as the heating medium for the reactors.Approximately, 800 kgs/hr of superheated steam at 250° C. and a pressureof 15 kg/cm² is estimated. The steam is used to heat the material in thereactors upto 185° C.

Compressed air (20) is arranged to pressurize the pressure reactor at 10kg pressure. Compressed air is also used for pneumatically activatedcontrol valves.

A buffer storage tank for compressed air water compressed air with 5000litres capacity is provided to facilitate uninterrupted supply ofcompressed air to the plant units. Suitable dryer arrangement isprovided to supply dry compressed air (20) to the control valves.

Modes of Operation:

According to the invention, three modes of operation can be madeavailable for the leaching process. Any of the operation modes can befinally adopted and the process optimized for achieving the maximumreduction of ash content in the coal. The details of the three differentmodes of operation are shown in Annexure—1.

Final Product

The coal cake obtained after the final treatment and filtration in thesecond rotary vacuum filter (18) is collected separately. The coal cakeis expected to have a moisture content of about 20%. The cake is airdried to reduce the moisture and then bagged in 1 tonne jumbo bags. Thebags are then stored in product storage area for further transportationto the user. The coal cake samples from each batch are collected andanalyzed for various physical, chemicals, rheological, petrographicaland other special analysis.

We claim:
 1. An improved beneficiation process to produce low ash cleancoal from high ash coals, comprising the steps of: (a) feeding raw coalto a continuous ball mill with size reduction via a primary crusher, (b)screening the crushed coal output from the ball mill; (c) transferringthe crushed and screened fine coal to a bunker having load cells forweight-monitoring of the fine coal including a vibratory device forsmooth inflow, and a rotary feeder device for easy outflow from thebunker; (d) preparing a coal slurry from the fine coal received inbatches from the bunker in a slurry preparation tank having means forstirring, and provided with process water controlled via a controlvalve, the tank being further supplied with compressed air, the preparedcoal slurry being discharged from the tank to a first reactor, the firstreactor being maintained at a temperature between 85° C. to 90° C.; (e)preparing an alkali solution with a predetermined concentration fromNaOH and water in a first tank; and discharging the prepared alkalisolution to said first reactor; (f) carrying-out a leaching reactionbetween the coal slurry and the aqueous alkali solution inside the firstreactor for about 2 to 5 hours by varying the speed of a stirrer insidethe first reactor to form a reaction mixture; (g) transferring thereaction mixture from the first reactor to a second reactor eitherdirectly or through a first rotary drum filter for filtration andwashing to produce a filter cake and a filtrate, the filtrate beingtransferred to a separate storage tank, the filter cake beingcontinuously washed by using sprayed water and transferred back to acoal slurry preparation tank; (h) preparing a diluted acid solution in asecond tank by supplying therein concentrated acid and process water viafeed lines having control valves and flow sensors, the second tank beingprovided with at least one level sensor, and at least one stirrer, theprepared acid solution being delivered to a second reactor, the secondreactor having a stirrer; (i) preparing a diluted alkali solution in athird tank having at least one first feed line for delivery of recycledalkali, a hopper with load cells, control valves, flow sensors, levelsensors, a stirrer, and at least one second feed line for a supply ofprocess water, the produced alkali solution including recycled alkalibeing delivered to the second reactor; (j) preparing a slaked lime in afourth tank for adding the slaked lime with the recycled alkali alongwith the fresh alkali; (k) feeding a coal slurry prepared from saidfilter cake into the second reactor for washing the alkali-treated coalwith acid at room temperature, and transferring to a second filtrationunit for filtration; (l) transferring a product after filtration to athird reactor; (m) carrying-out different leaching reaction sequences inthe first, second, and third reactor; (n) transferring the treatedslurry to a second filtration unit after completion of the reaction fromthe third reactor, producing a filter cake and a filtrate; (o)transferring to a fifth tank the finally treated coal filter cake fordrying, the dried filter cake being taken up for physical, chemical,rheological, and petrographical analysis; and (p) neutralizing thefiltrate discharged from the second filtration unit with lime forprecipitation and transferring the rest of the filtrate to an evaporatorfor concentration of NaOH.
 2. The process according to claim 1, whereincoal crushed in batches of 500 kg to −30 or −72 BS mesh size or a finecoal obtainable from floatation circuit coal is used.
 3. The processaccording to claim 1, wherein the coal slurry is prepared in the form ofa dough, a cake, or a thick slurry by mixing the prepared coal withprocess water.
 4. The process according to claim 1, wherein the alkaliis sodium hydroxide (NaOH).
 5. The process according to claim 1, whereinthe acid is hydrochloric acid or sulphuric acid.
 6. The processaccording to claim 1, wherein the alkali concentration is 10-50% and theacid concentration is 10-20%.
 7. The process according to claim 1,wherein the treatment steps in the reactors comprise a treatment of thefeed coal in an aqueous alkaline solution at an elevated temperatureunder atmospheric pressure and elevated pressure followed byreaction/extraction with an aqueous acidic solution at atmospherictemperature and pressures.
 8. The process according to claim 1, whereinthe stirrer speed inside the reactors is maintained at around 200 rpm.9. The process according to claim 1, wherein process water having notmore than 10 ppm hardness is used.
 10. The process according to claim 1,wherein pressure in the first reactor is created by the compressed airsupplied at a pressure of 10 kg/cm².
 11. The process according to claim1, wherein the coal filter cake obtained after final treatment contains20% moisture and is air dried to reduce the moisture.
 12. The processaccording to claim 1, wherein coal ash is reduced up to 4-5% by weightwith a yield of around 75-80% after leaching.
 13. The process accordingto claim 1, wherein different leaching sequences are used in thereactors.
 14. The process according to claim 1, wherein a super heatedsteam is used to heat the coal slurry in the first reactor.